This invention relates to ceramic block filters with high performance in a small package. More specifically, the present invention relates to a new design for a high performance dielectric ceramic filter that is smaller than conventional filters with comparable performance specifications and which is designed to reduce second and third order harmonics.
A ceramic body with a coaxial hole bored through its length forms a resonator that resonates at a specific frequency determined by the length of the hole and the effective dielectric constant of the ceramic material. The holes are typically circular, or elliptical. A dielectric ceramic filter is formed by combining multiple resonators, each of the resonators passing through the entire ceramic block, from the top surface to the bottom surface, such that the depth of each hole is the same as the axial length of the filter. The design choice for a specific axial length of a filter depends on the desired frequency and the dielectric constant of the selected ceramic.
The ceramic block functions as a filter because the resonators are coupled inductively and/or capacitively between every two adjacent resonators. These couplings are formed by the electrode pattern which is designed on the top surface of the ceramic block and plated with a conductive material such as silver or copper. More specifically, and with reference to FIGS. 1A-D, a ceramic block 101 is shown with two holes 103 and 105. All surfaces, except for the front open face 107 through which the two holes 103 and 105 extend, are plated with silver. Due to the size of the holes, their proximity and the conductive coating, the two holes 103 and 105 are inductively and capacitively coupled to each other. However, block 101 will not perform as a filter because these couplings cancel each other out.
To form a filter, a pattern of conductive material is printed on face 107, as shown in FIG. 1B. In this embodiment the patterns A and Axe2x80x2 enhance the capacitive coupling between holes 103 and 105. While the capacitive coupling is enhanced, the inductive coupling remains substantially unaffected. This is because inductive coupling is mostly a function of the hole diameter, shape and spacing between holes. These parameters are the same in FIGS. 1A and 1B.
The capacitive coupling can be regulated in FIG. 1B by adjusting parameters L and G. By decreasing G or increasing L, the capacitive coupling is strengthened. The capacitive coupling can also be weakened such that the inductive coupling is stronger, by printing line M on open face 107. The simple line M in FIG. 1C has a greater diminishing effect on the capacitive coupling of the block filter 101, than the broken line Mxe2x80x2 of FIG. 1D.
Ceramic filters are well known in the art and are generally described for example in U.K. Patent No. GB2163606 which is hereby incorporated by reference as if fully set forth herein.
With respect to its performance, it is known in the art that the band pass characteristics of a dielectric ceramic filter are sharpened as the number of holes bored in the ceramic block are increased. The number of holes required depends on the desirable attenuation properties of the filter. Typically a simplex filter requires at least two holes while a duplexer (having a transmitter filter and a receiver filter) requires more than three holes. This is illustrated in FIG. 2 where graph 10 represents the filter response with fewer holes than graphs 12 and 14. It is apparent that graph 14 which is the response of the filter with the most holes, is the sharpest of the three responses shown. Referring to FIG. 3, it can be seen that the band pass characteristic of a particular dielectric ceramic filter is also sharpened with the use of trap holes bored into the ceramic block. Solid line graph 21 represents the response of a filter without a high-end trap. Dashed line graph 23 represents the response of the same filter with a high-end trap.
Trap holes, or traps as they are commonly referred to are resonators which resonate at a frequency different from the primary filter holes, commonly referred to simply as holes. They are designed to resonate at the undesirable frequencies. Thus, the holes collect the desirable frequencies while the traps remove the undesirable frequencies, whether low end or high end. In this manner the bandwidth characteristic of the filter is defined, i.e. high pass, low pass, or band pass.
Block filters give rise to second and third harmonics which cause electrical problems, including noise, in many applications including cellular telephones. The designer and user of a bandpass filter generally expects that the filter will have a response only within the range of frequencies for which it is designed. Filters that have second and third order harmonics, however, have responses for one or more ranges of frequency above the bandpass of the filter. Specifically, the third harmonic typically arises since the quarter wavelength resonantors used in block filters also resonate at three-quarter wavelengths, i.e. the third harmonic. The second harmonic typically is a consequence of the structure of the block filter.
While the second harmonic is suppressed by controlling the dimensions of the block filter, the third harmonic is typically controlled with low pass filters to block these higher ranges of frequency. Alternative methods include the use of step impendance holes in the filter.
With both low pass filter and step impedance solutions to attenuating the higher order harmonics, the block filters involve additional complexity, thereby increasing the cost of the filter. Furthermore, with low pass filters, either a separate low pass filter is needed on the PC board, or additional holes are used to block second and third order harmonics. As a result, either the size of the filter increases as compared with a similar block filter without the additional low pass filter, or additional room on the PC board is required for the additional low pass filter. This is of serious concern since one of the principle purposes of a block filter is to provide a high performance filter in a package as small as possible. Accordingly, it is desirable to design a ceramic filter that will reduce the effects of second and third harmonics without increasing the size or cost of the filter.
A metallized belt pattern is printed on the top face of a ceramic block filter which ordinarily has the printed conductive pattern, and connected to ground at the bottom of the filter. The metallized pattern at the side of the ground has an unmetallized line along at least one edge of the filter. This combination of the metallized belt and unmetallized line acts as a transmission line whose ends are short circuited and controls the third harmonic. The width of the metallized pattern and unmetallized line is a matter of design choice to attenuate second and third harmonics.